The proposed research deals with the representation and processing of magnitude and order information. Theoretical interest centers on the processes by which people can compare two stimuli on the basis of subjective magnitude or relative order in a linear series. Major questions include: a) How is the judgement process affected by the form of the comparative question, or by an explicit reference stimulus? b) how similar are the processes involved in judgements based on memory vs. perception? c) What is the relationship between order and magnitude information? d) What role does magnitude information play in semantic processing? A number of models of judgement processes are discussed and critically evaluated. The primary experimental strategy is to obtain new sources of data relevant to the evaluation of possible comparison models. Major independent variables include the magnitude difference between a pair of symbols, the congruity of the question and the magnitude of the stimuli, and the serial position of stimuli in a linear array. Reaction time, accuracy and rated magnitude difference are used as dependent measures in experiments that manipulate the explicit and implicit reference points used in the comparison process. Further experiments involve manipulations of variables that may affect the relative salience of magnitude vs. order information. In addition, other techniques, such as detailed analyses of recognition confusions, are used to investigate the semantic representations of ordered concepts. Further experiments explore alternative decision strategies in mental comparison and mental arithmetic tasks.